The invention relates to tape transport capstan and reel tension servo circuits and particularly to a feedforward circuit which applies the capstan drive current to the reel tension servo circuits to force the same resultant tape acceleration from the reel motors, thereby minimizing tension arm movement and tape storage in the tension arms.
In most audio and video tape recorders, and particularly in state-of-the-art broadcast video tape recorders, a capstan and associated capstan servo circuit controls the tape movement, including acceleration and deceleration. In such broadcast recorders, reel tension servo circuits also are used to control the reels, and include tension arms in the circuit to control the tape tension as tape is pulled from the supply reel to the takeup reel, and vice versa. It follows that if an acceleration mode is requested via a reference command, the capstan servo circuit increases the capstan drive current, thereby increasing the rotational velocity of the capstan and accelerating the movement of the tape. Obviously, the increase in the tape velocity demands a corresponding increase in the rotational velocity of the reels. As well known in the art, however, the inertia of the capstan is much less than the inertia of the supply and takeup reels, particularly when a reel has a full tape pack. Furthermore, the response bandwidths of the reel tension servo circuits are much less than that of the capstan servo circuit, wherein accordingly, the reel motor drives always lag the capstan motor drive.
In addition, the reel tension servo circuits are responsive to their respective tension arm movements. Therefore it follows that a reel motor drive requires a tension arm displacement in order to produce an acceleration of the associated reel, or reels, commensurate with the acceleration imparted to the tape by the accelerating capstan. Also, due to the higher inertia and lower bandwidth, the reel cannot provide tape as rapidly as requested by the capstan acceleration. Without suitable compensation, the tension arms would be displaced beyond their usual operating range, and the reel tension servo circuits would undesirably lose control. To prevent the occurrence of such an undesirable operating condition, conventional transport control circuits include a tension arm limit detector circuit coupled between the reel tension servo circuits and the capstan servo circuit. The limit detector circuit generates a signal which instructs the capstan servo circuit to decrease the drive current to the capstan, thereby slowing the capstan and allowing the tension arms to remain within their operating range. However, decreasing the capstan drive current causes a corresponding decrease in the desired acceleration originally requested via the reference command, thereby unduly limiting acceleration, which in effect limits the transport system performance.
It may be seen therefore that the delay due to reel acceleration, delay due to capstan acceleration, and the delay due to the requirement for tension arms displacement, all combine to provide a decrease in system performance. In turn, there is a requirement for greater tape storage in the tension arms, which may cause undue mechanical constraints.
Accordingly, it would be highly desirable to provide a tape transport control system wherein the relatively rapid acceleration of the capstan is fully utilized to advantage, thereby improving the performance of the tape transport. In sophisticated broadcast tape recorders, rapid acceleration and deceleration such as when performing editing processes saves precious time and increases operating efficiency, an extremely important advantage in a broadcast video tape recorder.
The present invention circumvents the disadvantages of previous mention while providing a feedforward circuit which applies capstan drive current, or its equivalent, to the reel tension servo circuits whose associated reels are transporting tape. The additional drive current to the reel motors increases the rotational velocity of the reels thereby maintaining the same resultant acceleration between the tape driven by the capstan and the tape being transported between reels. Since the tape pack diameter affects the gain used in the feedforward circuit, a preselected average gain corresponding to the mean tape pack diameter is used for different cassette sizes to increase the feedforward circuit efficiency.
In one embodiment, a capstan composite error signal indicative of the desired reference command, such as "accelerate to shuttle speed", is supplied to a multiplexer, whose three outputs are individually selectable in response to a digital value indicative of the size of the tape cassette being used. The three outputs correspond to different cassette sizes of small, medium and large, and include selected resistances corresponding to the preferred average gain for the feedforward circuit for the corresponding cassette size. The selected output of the multiplexer is fed to an operational amplifier stage, is filtered, and also limited via a limiter circuit, to provide a takeup feedforward signal to the takeup reel of the transport. The signal is inverted by an inverter stage and is supplied as the supply feedforward signal to the supply reel.